1. Technical Field
Related subject matter is disclosed in a co-pending U.S. patent application Ser. No. 11/490,400, filed on Jul. 26, 2006, which is assigned to the same assignee as this application.
The present invention relates to pad layouts of printed circuit boards (PCBs), and more particularly to a pad layout of a PCB which can readily and reliably receive surface mounted components (SMCs) thereon.
2. General Background
In general, SMCs, including resistors, capacitors, inductors and so on, are connected to a printed circuit board by pads. The pads are arranged on the printed circuit board in a matched relationship with footprints of the SMCs. The pads have different configurations due to varied configurations of the footprints of the SMCs, such as an ellipse, a circle, a square, a teardrop, and so on. Generally, however, the pad is only capable of receiving a certain SMC.
Referring to FIG. 2, a pad layout of a printed circuit board (PCB) using computer software (such as Protel) is for arranging SMCs, such as a 0805-type component 3, and a 0603-type component 5. A size of the 0805-type component 3 is larger than that of the 0603-type component 5. A pair of pads 8 each with an ellipse configuration is arranged on the PCB for receiving the 0805-type component 3. A pair of pads 6 each with an ellipse configuration is arranged on the PCB for receiving the 0603-type component 5. A lined frame 2 of the pads 8 is bigger than a lined frame 1 of the pads 6. A width W1 of the 0805-type component 3 is greater than a width W2 of the 0603-type component 5. A length of the 0805-type component 3 is L1. An outer distance L3 between the pads 8 is equal to an outer distance between footprints of the 0603-type component 3. An inner distance L5 between the pads 8 is equal to an inner distance between footprints of the 0805-type component 3. An inner distance L2 between the pads 6 is equal to an inner distance between footprints of the 0603-type component 5. Wherein L3>L5>L2, and W1>W2. The configuration of footprints of the 0803-type component 3 and the 0603-type component 5 are different. Therefore, each of these different SMCs require a different corresponding pad. The 0805-type component 3 and the 0603-type component 5 each have a pair of footprints corresponding to the pads 8 and pads 6 respectively. However, space on the printed circuit board is limited. Too many pads will decrease available space for electronic components and traces on the PCB causing difficulties to a layout engineer when he tries to selectively arrange multiple components on the PCB. Warnings 4 generated by the layout program may be frequent during the layout process further affecting the layout engineer's work.
Referring to FIG. 3, showing another computer generated pad layout of a PCB, the PCB includes a pair of rectangular pads 7. Each of the pads 7 embraces configurations of the corresponding pads 8 and pads 6. An outer distance L33 between the pads 7 is equal to the distance L3 indicated in FIG. 2. An inner distance between the pads L23 is equal to the distance L2 indicated in FIG. 2. A width W13 of each of the pads 7 is equal to the width W1 indicated in the FIG.2. A length of each of the pads 32 can be concluded as (L33−L23)/2, that is, (L3−L2)/2. However, the surface area of the pads 7 is large, and during the soldering process smaller SMCs have a tendency to float on the melted solder. This causes an untidy appearance of the PCB and can lead to short-circuiting on the PCB. Besides, to include all configurations of the corresponding pads 8 and pads 6 in the pads 7 may also cost more.
What is needed is a pad layout on a PCB which can readily and reliably receive surface mounted components (SMCs) on the PCB.